A critical element in the operation of air-borne and space-borne imaging systems is sensor calibration on the ground before launch or flight. The purpose is to provide a complete characterization of a sensor's spectral, spatial, temporal, radiometric and polarization response functions. This is true for radiometric instruments designed to measure scene radiance at multiple wavelengths (imaging radiometers) or those specifically designed to measure the polarization properties in the radiation from objects in the scene (imaging polarimeters). In both cases, laboratory measurements are generally performed to quantify the effects of the instruments polarized response achieving performance characterization or calibration in supporting their overall measurement objectives.
For imaging radiometers, polarimetric characterization is desired for each spectral channel. Ideally, their polarization response should be nonexistent with the radiometric response independent of whether any of the radiation recorded from a source is polarized or not. But in many cases, polarization responsivity becomes present through the use of optical components such as off-axis mirrors, mirror coatings, beam splitters or diffraction gratings.
In contrast, polarimeters require polarimetric calibration. Imaging polarimetry is dedicated to mapping the state of polarization across a scene of interest. The properties of polarized radiation (i.e. unpolarized, partially polarized or completely polarized) are generally described by a four-element real vector known as Stokes vector. The symbols S0, S1, S2 and S3 represent the four Stokes-vector elements. The first three elements describe linear polarization and are determined from a minimum of three images recorded through polarizing filters at different rotation angles. Generally, the elements are calculated from a set of orthogonal intensity terms recorded in four images of a scene each through polarizers oriented at angles of −45, 0, 45 and 90 degrees. The calibration process derives coefficients for transformation equations that convert the intensity measurements into Stoke-vector elements.
Calibration performance for remote sensing systems is generally best known when analyzed during ground testing. However, it is entirely possible that physical conditions within the imaging system (including any on-board calibrators) or in the atmosphere between the imaging system and the desired target may change from the calibration laboratory setting in such a way so as to skew the performance knowledge or calibration values. The calibration performance thus becomes suspect until validated after deployment.
Under operational conditions, validation requires the use of vicarious calibration sources that function independent of the previous laboratory testing or on-board calibrators. Prior art has consisted of: 1) vicarious polarimetric references established through field campaigns to characterize the polarimetric state of natural targets, 2) deploying large man-made non-Lambertian surfaces, 3) spotlights shining through polarizers, or 4) modeling scattered sunlight in the atmosphere. These approaches have either proven very costly or have achieved very limited accuracy and reproducibility. Thus, a need exists in the art for improved systems and methods for polarimetric calibration of remote sensors.